util/intelmetool: Add support for platforms without RCBA

Only try to unhide MEI if the PCI device wasn't found and
probe for RCBA before trying to use it.

Allows to run the utility on Skylake and newer hardware that
do not have RCBA any more.

TODO: Use sideband interface to unhide MEI.

Change-Id: I7926aa80b132d5be9fece0724516701d74dd4d3d
Signed-off-by: Patrick Rudolph <patrick.rudolph@9elements.com>
Reviewed-on: https://review.coreboot.org/25399
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Paul Menzel <paulepanter@users.sourceforge.net>
Reviewed-by: Philipp Deppenwiese <zaolin.daisuki@gmail.com>

util/intelmetool/rcba.c

@@ -22,7 +22,7 @@
 static const int size = 0x4000;
 
 /* Returns the physical RCBA base address or zero on error. */
-static u32 get_rcba_phys(void)
+u32 get_rcba_phys(void)
 {
 	struct pci_access *pacc;
 	struct pci_dev *sb;